Exploring the ground in the circuit -- Overcoming electromagnetic interference

On printed circuit boards, power cords and ground wires are the most important. The main method to overcome electromagnetic interference is grounding.

For the dual panel, the ground line layout is particularly exquisite. By using the single point grounding method, the power supply and ground are connected from both ends of the power supply to the printed circuit board. The power supply is one point, one point. On the printed circuit board, there must be multiple return ground wires, which will be aggregated to the point of the Back-to-power supply. This is called single point grounding. The opening of analog, digital, and high-power devices means that the cables are separated and finally collected to the ground point. When connected to a signal other than a printed circuit board, a shielded cable is usually used. For high-frequency and digital signals, both ends of the shielded cable are grounded. Shielded Cables for low-frequency analog signals. It is better to ground one end. Circuits that are very sensitive to noise and interference, or circuits with extremely high-frequency noise, should be shielded with metal masks.

Understanding of one point grounding and multiple points grounding:

1. Single Point of Grounding-single point of grounding is that in the entire system, only one physical point is defined as a grounding reference point, and all other points to be grounded are connected to this point. Single point grounding is suitable for low frequency circuits (less than 1 MHz ). If the operating frequency of the system is very high, and the operating wavelength is comparable to the system grounding lead length, the single point grounding mode will be problematic. When the length of the local line is close to 1/4 wavelength, it is like a short-circuit transmission line of a terminal. The current and voltage of the ground line are distributed in a standing wave, and the ground line is changed to a radiation antenna, it cannot play the role of "land. In order to reduce Grounding Impedance and avoid radiation, the length of the ground line should be less than 1/20 wavelength. In terms of power supply circuit processing, you can generally consider single point grounding. For PCB with a large number of digital circuits, it is generally not recommended to use single point grounding because of its rich high harmonic.

2. Multi-Point Grounding means that each grounding point in the device is directly connected to the grounding plane closest to it to minimize the length of the grounding lead. Multi-point grounding is suitable for circuits larger than 10 MHz. Single Point Grounding and multi-point grounding can be used in digital-analog hybrid circuits.

3. Generally, ground at one point is used for simulation. Numbers must be grounded at multiple points. Separate large currents from small ones.

4. one point of grounding: Simply put, the ground of all unit circuits is connected to one place, and each unit circuit pulls a ground line. The ground of each unit circuit is in either AC or DC state, there is no shared ground principle

(1) Application Principles of One-point grounding and multi-point grounding

The high-frequency circuit should be grounded at the nearest point, and the low-frequency circuit should be grounded at one point.

(2) Communication and signal locations cannot be shared

(3) Comparison of float and grounding

The full-host float method is simple, but the insulation resistance between the whole-host and the ground cannot be less than 50 MΩ.

(4) digital location

The ground wire in the printed board should be a mesh, and other wiring should not form a loop.

(5) Simulated location

Generally, float isolation is used.

(6) power location

It should be separated from the small signal ground and connected to the DC Ground.

(7) Signal Location (sensor location)

The location of the sensor is generally grounded at a point with a 5-Ω conductor (Grounding Resistance), which is not flushed.

(8) shielding Area

This type of ground is used to shield the electric field.

................................ I would like to cite a good article and thank the original author (Wu Xiaoqing )........................

Grounding is the most basic content in circuit design, but almost no one can say it clearly. Almost every training and communication will be asked, "Is there a general grounding method for reference ?" If you want to know the answer to this question, please continue to read it. First, I will give you an answer: "No ". So what should we do? We can't always use sensory words like a Chinese chef, for example, "a little", "slightly yellow color", and "microfocus" for cooking, of course not. In order to better understand the grounding techniques and methods, we will not pay attention to any text skills below, but will reveal the essence of the grounding problem.

The grounding method refers to the grounding function of the grounding purpose, so which grounding method is adopted depends on what type of ground and what the purpose of such ground is. These two problems have been solved, the grounding method can be easily implemented. The purpose of grounding determines the grounding method. Different purposes of the same circuit may adopt different grounding methods.Remember this point of view. For example, the same circuit is used on portable devices, and static electricity accumulation and discharge are not allowed. The purpose of grounding is to balance the potential of the ground. For non-movable devices, there are usually safe grounding measures, the ground for electrostatic discharge is designed to have low conduction impedance, especially for high-frequency conduction impedance of peak pulses.

The points of attention in the explanations are divided into several independent points of view,Although the content of each article is simple, it is recommended that you read it over and over again N times, like facing a cup of good tea, so that the heartbeat is under the Status of 60 BPM, the fine products, the feeling of beauty and connotation. Then, we can understand the profound truth from simple words.

1. There are four types of grounding based on performance:

Safe Grounding, working grounding (digital, analog, power device), anti-surge grounding (lightning surge, power-on surge), anti-static grounding. As mentioned in the previous documents, "the purpose of grounding determines the grounding method", which refers to its functions. Basically, all grounding can be attributed to these four types. Before each grounding, you must determine which grounding belongs.

2. The goal of grounding is low ground impedance, stable ground, and balanced ground.

It is easy to understand low ground impedance, just use a rough cable, but there is a problem that cannot be ignored, for example, I am grounded through a large inductance, if the frequency of the local current running on the ground is 0.00000001Hz, the inductive effect of this large inductance is not obvious, which is equivalent to direct grounding, but if the fluctuating current is Hz, the inductance = j ω L = j 2 π f L is very large. In this case, it is equivalent to the high-frequency grounding. You may say, "Come on, you know, who will use a large inductance for grounding? The first is that there will be such a method in a certain state, and the second is that even if you do not connect an inductor like this, the cabling inductance of common Cables cannot be ignored at high frequencies. In summary, low frequency Grounding is less than high frequency grounding. That is to say, the ground with low impedance should be analyzed whether it is a high-frequency or low-frequency ground.

It is easy to understand the ground stability. In general, if the ground impedance is low enough, it is easy to let the ground electricity flow out and put, and there will be no pressure drop on the bottom line, just like an ultra-large capacitor, the ocean of charge has an infinite and broad mind, and it is not surprising when many incoming traffic comes. The ground balance is easy to ignore. For a signal, the useful part is the pressure difference between the two lines. If the ground line is drifting, the pressure difference between the two lines increases or decreases equally, that is, the differential mode voltage value remains unchanged, and the common mode voltage changes. In fact, the circuit function is implemented as usual. It's like a sea of water, you are 3 cm higher than me, standing on the ship, the ship is up, you are still 3 cm higher than me. This situation is often used in Electrostatic protection. An electrostatic pulse hits the circuit board through the air. For a local circuit, the distance is different, which will certainly lead to the pressure difference caused by electrostatic induction. At this time, if a metal board is separated, even if the metal board is floating, the circuit board behind the metal board will sense a uniform electric field, although the interference still exists, but at least the circuit is basically balanced. Of course it would be better if the metal sheet was grounded. Of course, the common mode voltage is generally not maintained, because the impedance of the transmission line is uneven, it is often converted into Differential Mode Voltage interference, the problem of ground balance is best not to let us face, but when there is no way, for example, if a floating device has to be subjected to electrostatic impact on the circuit board, the local balance should be considered during protection.

3. Co-ground impedance coupling interference

Ground impedance coupling interference is the core problem that must be faced every day in the ground, and it is almost unavoidable. Just like in the movie theater, when you came out of Hall 1 in the innermost part of the cinema, few people came smoothly. Suddenly Hall 2 also went off, and the channel became crowded, moving on again, it broke down. Hall 3 was entering the audience. Suddenly, the flow of people fluctuated. This is in line with the common ground impedance. The channel is equivalent to the ground line and the human is equivalent to the current. If the number of people in Hall 1, Hall 2, and Hall 3 is similar, the interaction between them is not very great. However, if Hall 3 is a hall, the number of people in Hall 1 and Hall 2 is many times, personnel entering and exiting Hall 3 will have a great impact on the flow speed of personnel in Hall 1 and Hall 2. Guests of the first, second, and third boats will all go through this road to form a common ground impedance.

For example, in figure 1, the resistance of the RAB section is the common ground impedance, and the local current I/O, IA, and Id flowing through this section will affect each other; if the difference between the three currents is large and the difference is 1-2 orders of magnitude, the influence between them cannot be ignored, in particular, a weak current branch is used for quantitative measurement, amplification, or AD conversion. Figure 2 isolates the effect of ID on the other two paths; figure 3 shows that all three currents are isolated.

4. General grounding method

This title uses the word "Greater" For some reason, because the general grounding method does not exist at all. This is just a basic model, it also needs to be flexibly handled according to the actual situation, just like the language, the same sentence "you hate", when talking in different tone, the information transmitted is very different. The basic idea is to ensure independent connection of the security protection area, work digital, work analog, work power, lightning surge, and shield in the design, finally, in the joint commissioning of the system, according to the problems to be solved between different regions, that is, according to the purpose of grounding, the connection methods of these regions are handled according to the following Connection Methods:

A-Direct Connection of yellow-green wires between the Earth

It is best to understand this method, that is, simply making two low-impedance connections reliably. However, it should be noted that this method is only applicable to the connection between medium and low frequency signal circuits. This type of wire has a certain degree of wiring inductance and wiring resistance. It is equivalent to low-frequency Grounding for high-frequency fluctuating ground current, high-frequency high-impedance grounding, basically cannot achieve high-frequency reliable conduction.

B-ground-wide flat cable for Direct Connection

Flat cables are mainly used to solve the problem that cannot be solved by the direct connection of the above wires. The grounding cable of the electrostatic test table does not need a straight line. It can realize ground impedance to ground conduction at high frequencies.

C-ground-large resistance connection between regions

A large resistance is characterized by a pressure difference between the two ends of the resistance, resulting in a very weak Conduction Current. After the local charge is discharged, the pressure difference between the two ends is equal to 0 V, this feature is best presented when you want to increase the charge, but do not want to decrease the charge quickly. Production Site anti-static mats, the on-resistance is generally-euros, this is the purpose. The anti-static mat is equivalent to the large resistance between the ground of the circuit board and the Earth protected. The characteristics of the capacitance connection between C and ground are DC cutoff and AC conduction. This method can be considered when this type of function is to be realized. For example, for a product powered by a switching power supply, the housing is connected to the protection ground, and the ground on the circuit board is messy and fluctuating, but there is no place to let it go, the DC power supply is connected to a large capacitor between the 24 V, 12 V, and 5 v dc power supplies and the protection grounding. The fluctuation can be dropped, but the DC composition can ensure stability, in this case, if the stability of the protected area and shell is not guaranteed, the effect may be counterproductive.

D-ground-space magnetic beads

Here, the characteristics of the magnetic beads need to be clarified. Many engineers often equate the magnetic beads with the inductance, which is a fundamental mistake. Magnetic Beads are equivalent to a frequency-changing resistance, which represents a resistance property and a consumable property, and an inductance is an energy storage property, which is equivalent to pin peak fill. Therefore, the ground of the Cross-magnetic beads is usually in a fast and small current fluctuation State, because the magnetic beads will be saturated, the current is too large, and it cannot be consumed. It is generally used between the ground of a weak signal.

E-ground-inductance connection between locations

The inductor can suppress the changes in the circuit status. Through the connection of the inductor, the pin peak can be filled in. For the ground with large current fluctuations, the cross-connected inductor can solve this problem.

F-ground-small resistance connection between ground

The problem to be solved by the small resistance is that a damping is added, which hinders the overrush of the ground current and slows down the rising edge of the impact current when the current changes, it is equivalent to reducing the matching resistance of the crystal oscillator output end and the bus output end.

5. Safe and lightning-proof connection

Because lightning surge and safe ground current are generally far greater than the damage of signal current to people, the two grounding suggestions are separately connected to the Earth, in a single point of connection on the real Earth, especially the lightning protection grounding.

This article takes about a month, and all kinds of ideas have been hovering around, but there is no starting point. On me, it also confirms the relationship between the problem and us, the most familiar and unfamiliar, it is the simplest and most complex, and the easiest to get on the road and the hardest to get to the destination. We hope that through a rough summary, we can provide a cloudification ladder for the Grounding Design on the fog end in the cloud, so that the Grounding Design can truly be connected to the ground gas.